Does canopy nitrogen uptake enhance carbon sequestration by trees?

Abstract

Temperate forest 15N isotope trace experiments find nitrogen (N) addition‐driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply 15N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional 15N additions typically trace mineral 15N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/15N ha−1 yr−1 to Sitka spruce saplings. We compared tree and soil 15N recovery among treatments where enrichment was due to either (1) a 15N‐enriched litter layer, or mineral 15N additions to (2) the soil or (3) the canopy. We found that 60% of 15N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of 15N applied to the soil was found in these pools. 15N recovery from litter was low and highly variable. 15N partitioning among biomass pools and age classes also differed among treatments, with twice as much 15N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from 15N applied to the soil, scaled to real‐world conditions, was 43 kg C kg N−1, similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N−1. Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Global forest carbon uptake due to nitrogen and phosphorus deposition from 1850 to 2100

“…We estimated that ▵C/▵N was 24 kg C kg N −1 , which is lower than the estimate by Nair et al. (), but close to the estimate (11.5–39.8 kg C kg N −1 ) by de Vries et al. (), because Nair et al.…”

“…Here we calculated the C response to N deposition based on the fate of deposited N at a timescale of 10-20 years following de Vries et al (2014). We estimated that MC/MN was 24 kg C kg N À1 , which is lower than the estimate by Nair et al (2016), but close to the esti- anthropogenic P deposition. This suggests that the effect of P will be less significant if the high C response to N deposition due to direct use of N by the canopy can be confirmed by more evidences.…”

“…(), because Nair et al. () did not account for loss of N in the ecosystem via denitrification and leaching. We increased our estimate of ∆C N dep by 2.6‐fold as indicated by Nair et al.…”

“…Dezi, Medlyn, Tonon, and Magnani (2010) found that the net ecosystem production would be increased by 58% under a hypothesis that canopy N uptake can directly stimulate photosynthesis relative to without canopy N uptake. Nair, Perks, Weatherall, Baggs, and Mencuccini (2016) suggested that accounting for canopy N uptake could lead to an increased C response to N deposition (MC/MN) from 43 to 114 kg C kg N À1 (by 2.6-fold) through a welldesigned mesocosm experiment. It should be noted that the experiment by Nair et al studied the response to N deposition at a timescale of one year and it is likely that N in the aboveground biomass can be re-allocated in plants at a longer timescale.…”

Fate of N additions in a multiple resource‐limited Mediterranean oak savanna

“…Contrary to our expectation, the highest plant recovery was in March 2017, just 24 h after label was applied. High recovery immediately after application suggests that much of the label was taken up through leaf tissue (Sparks 2009, Nair et al 2016, meaning that plants were never in competition with microbes for this N. The slight increase of label recovery in plant tissues under tree canopies from fall 2017 to spring 2018 could indicate the use of N leached to soil below five cm.…”